WASHINGTON — President Trump on Monday denounced the suspected chemical weapons attack that killed dozens of people in Syria over the weekend as a “barbaric act’’ and said he will ‘‘forcefully’’ respond to it.

Trump spoke in the Cabinet Room of the White House during a meeting Monday evening with his top military leaders, including Defense Secretary Jim Mattis and General Joseph Dunford, the chairman of the Joint Chiefs of Staff.

He did not discuss the timing of the response or whether it would involve a military retaliation.

“We’re talking about humanity and it can’t be allowed to happen,’’ Trump told reporters earlier in the day, suggesting that a response would be forthcoming soon. “We’ll be making that decision very quickly.’’

Calling the attack “heinous’’ and “atrocious,’’ the president suggested that Syria’s patrons in Russia and Iran may also be responsible and seemed to imply that he would take action of some sort to punish them as well.

“If it’s Russia, if it’s Syria, if it’s Iran, if it’s all of them together, we’ll figure it out and we’ll know the answers quite soon,’’ he said. “So we’re looking at that very strongly and very seriously.’’

Asked if President Vladimir Putin of Russia, with whom Trump has sought to forge a friendship, bears responsibility, the president said: “He may and if he does it’s going to be very tough, very tough. Everybody’s going to pay a price. He will, everybody will.’’

The United Nations Security Council held an emergency meeting Monday to discuss the Syria attack but Russia, as a permanent member, holds a veto that could block any action by the world body.

Nikki Haley, the American ambassador to the UN, said, ‘‘the United States is determined to see that the monster who dropped chemical weapons on the Syrian people held to account.’’ Haley didn’t identify the ‘‘monster’’ but her sharp words appeared aimed at Syrian President Bashar Assad.

The Organization for the Prohibition of Chemical Weapons, the international watchdog on chemical weapons, said it has opened an investigation of the suspected poison attack.

As international condemnation grew over the poison attack, Syria and its main ally, Russia, blamed Israel for airstrikes on a Syrian air base early Monday that reportedly killed 14 people, including four Iranian fighters.

The timing of the airstrikes in central Homs province, hours after Trump said there would be ‘‘a big price to pay’’ for the chemical weapons attack, raised questions about whether Israel was acting alone or as a proxy for the United States.

Israel did not comment on Monday’s missile strike. It typically does not comment on its airstrikes in Syria, which have been numerous in Syria’s civil war.

The White House is feeling pressure from allies to act on the chemical attack. President Emmanuel Macron, who spoke Sunday with Trump by telephone, has repeatedly declared that the use of chemical weapons by Syria’s government would be a red line and pledged to strike weapons sites connected to such attacks.

After the phone call, the White House said “both leaders strongly condemned’’ the attack and agreed that the Assad government “must be held accountable.’’ They vowed to “coordinate a strong, joint response.’’

The challenge for Trump’s Middle East policy came on a day when he was already facing a transition in his foreign policy team as his new national security adviser, John Bolton, arrived for his first day on the job and the president hosted the nation’s senior military leaders for dinner at the White House.

Bolton sat behind Trump during the Cabinet meeting but made no comment while reporters were in the room.

National security and military officers met Monday to discuss options, but defense officials would not say what specific military actions are on the table.

Mattis, speaking with reporters Monday, sounded a muted tone.

“The first thing we have to look at is why are chemical weapons still being used at all when Russia was the framework guarantor of removing all the chemical weapons,’’ Mattis said as he hosted the visiting emir of Qatar at the Pentagon. “And so, working with our allies and partners from NATO to Qatar and elsewhere, we are going to address this issue.’’

Asked if he would rule out airstrikes against Assad’s government, Mattis said, “I don’t rule out anything right now.’’

Two Arleigh Burke class guided-missile destroyers are located in the Sixth Fleet’s area of operations in the Mediterranean Sea and would be able to get within striking range within hours to days.

When Trump ordered the retaliatory strike against Syria for a chemical weapons attack at almost the exact same time last year, it was carried out by two destroyers firing 59 Tomahawk cruise missiles at Al Shayrat airfield, the suspected source of the chemical attack.

The strike targeted fighter jets, hardened aircraft shelters, radar equipment, ammunition bunkers, and sites for storing fuel and defense systems.

The chemical attack in the suburb of Douma over the weekend killed at least 49 people and raised the temperature of an already simmering relationship between the United States and Russia. Putin has troops in Syria propping up Assad’s government.

Russia has rejected the conclusion that Syria’s military was behind the chemical attack, asserting that it was staged by militants to falsely blame the government and justify a US strike against Assad’s regime.

Foreign Minister Sergey Lavrov told reporters in Moscow that despite Trump’s comments last week that he wanted to withdraw US troops from Syria, the United States was actually seeking to entrench itself in the country.

“The US is taking steps not to leave as President Trump said, and leave Syria for others, but to establish a foothold there for a very long time,’’ Lavrov said.

Trump dismissed the Russian and Syrian denials. “They’re saying they’re not’’ responsible, “but to me there’s not much of a doubt,’’ he told reporters.

Trump said that Syria was not allowing any independent inspection of the attack site. “If they’re innocent, why aren’t they allowing people to go in and prove’’ it, he asked.